[CUDA][HIP] Fix empty ctor/dtor check for union

union ctor does not call ctors of its data members. union dtor does not call dtors of its data members.
Also union does not have base class.

Currently when clang checks whether union has an empty ctor/dtor, it checks the ctors/dtors of its
data members. This causes incorrectly diagnose device side global variables and shared variables as
having non-empty ctors/dtors.

This patch fixes that.

Differential Revision: https://reviews.llvm.org/D79367